Path: utzoo!attcan!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!mcsun!ukc!edcastle!mfg
From: mfg@castle.ed.ac.uk (M Gordon)
Newsgroups: comp.unix.wizards
Subject: Re: Determining one's own IP address.
Message-ID: <1384@castle.ed.ac.uk>
Date: 12 Dec 89 13:14:17 GMT
References: <601@bmers58.UUCP> <4429@ur-cc.UUCP> <604@bmers58.UUCP>
Reply-To: mfg@castle.ed.ac.uk (M Gordon)
Organization: Edinburgh University Electrical Engineering
Lines: 55

In article <604@bmers58.UUCP> davem@bmers58.UUCP (Dave Mielke) writes:
>In article <4429@ur-cc.UUCP> leadley@uhura.cc.rochester.edu (Scott Leadley) writes:
>>	Assuming (a lot of things, but primarily that) you wish to do this from
>>the shell command line and that you know the network interface name:
> 
>I would like to be able to determine my local IP address without
>involving a hosts file or yp lookup, i.e. from memory, from within a c
>program.

Here's a shortened version of a program I wrote (This bit is very similar to
ifconfig). It prints out the addresses for all a machines interfaces. You
should have no trouble modifying it for what you want.

--------------------------------------------------------------------------------
#include <sys/types.h>
#include <sys/socket.h>
#include <netinet/in.h>
#include <net/if.h>
#include <sys/ioctl.h>
#include <stdio.h>

#define MAX_INTERFACES 10

main(argc,argv)
int argc;
char *argv[];
{
    struct ifconf ifc;
    struct ifreq *ifreq;
    struct sockaddr_in addr;
    char ifbuf[MAX_INTERFACES*sizeof(struct ifreq)];
    int s,n,i;

    s=socket(AF_INET,SOCK_STREAM,0);

    ifc.ifc_len=sizeof(ifbuf);
    ifc.ifc_buf=ifbuf;

    if (ioctl(s,SIOCGIFCONF,&ifc)==-1)
	exit(1);

    for (n=ifc.ifc_len/sizeof(struct ifreq),ifreq=ifc.ifc_req;n>0;n--,ifreq++)
    {
        bcopy(ifreq->ifr_addr.sa_data,&addr.sin_port,14);
        printf("%s at %-18s ",ifreq->ifr_name,inet_ntoa(addr.sin_addr));
    }
}
--------------------------------------------------------------------------------


Michael
-- 
Michael Gordon - mfg@uk.ac.ed.castle OR mfg@uk.ac.ed.ee

You can't have everything - where would you put it? -- Steven Wright